# User:Alex G. Benedict/Notebook/Physics 307L: Junior Lab/Speed of Light Lab

# Speed of Light Lab

^{SJK 21:09, 25 October 2010 (EDT)}This lab was performed by myself and my partner Joseph Frye User:Joseph_Frye on September 27, and October 4, 2010. I would like to thank Brian Josey and Kristin Harriger, for their advice on data collection.

### Set Up

There was actually not any set up required for this lab, since it was already assembled in the senior laboratory. For instructions on how to set up the lab see Dr Gold's manual, linked the last lab notebook or the Phys 307L website.

Equipment:

Tektronix Oscilloscope (Model TDS 1002)

Bertan Power Supply (Model 215, 3000V, 5mADC)

Canberra Delay Module (Model 2058)

Ortec TAC/SCA Module (Model 567)

Harshaw NIM Bin (Model NQ-75)

Harrison Laboratories Power Supply (Model 6207A, 160V, 0.2A)

Photomultiplier Tube (PMT)

LED circuit

### Procedure

^{SJK 21:08, 25 October 2010 (EDT)}The procedure for this lab consisted of starting the LED at some arbitrary distance from the photomultiplier, then moving it towards the photomultiplier at 10cm increments. Every time it was moved forward we tried to ensure that the oscilloscope was measuring about the same intensity for the light received by the photomultiplier tube by twisting the photomultiplier tube to adjust the angle of the polarizing filter.

On the first day we were keeping track of the distance from the PMT, and recording data using 25cm increments. On the second day we changed our procedure to work with arbitrary distances from the PMT. Instead of keeping track of the distance of the LED from the photomultiplier we just made note of the relative change in the distances, believing that the slope of the line is the most important part of the data since that should be the speed of light. The voltage measured is representative of the time it takes to reach the PMT. We know from the previous work of Josey and Harriger that the conversion factor of voltage to ns is 1V=10ns.

^{SJK 20:10, 25 October 2010 (EDT)}

### Data & Analysis

Here is the spreadsheet of the data. {{#widget:Google Spreadsheet |key=0AsLE8iWrtjlSdG55d1hPNVp1c20waXMtLXhEX3F5U0E |width=1200 |height=600 }}

^{SJK 21:06, 25 October 2010 (EDT)}The data from the first day is not the best since we were recording too few data points for each trial, and were using a bad technique, it is plotted below vs the best fit line through the points.

The slope of the line in the picture is 0.20079999999999 +/- 0.1258206130436 which actually contains the speed of light(about 0.3 in those units) within the uncertainty. So we known with about 68% confidence that the speed of light is in the interval [7.4,32.59] cm/ns.

For the next day we changed our technique to be distance arbitrary. The data for this had lower uncertainty, since we were using more points. The average slope for this data was -3.26848484848484 +/- 0.24476146068253, where the uncertainty is taken to be the maximum of the the uncertainties of each fit**The minus sign is irrelevant and comes from the fact that the distance from the LED to the PMT was decreasing, so with 68% confidence we know the speed of light is within the interval [30.24,35.13] cm/ns.**

^{SJK 21:03, 25 October 2010 (EDT)}A sample plot from this day is given below for trial 8:

The actual value of the speed of light is 29.9792458 cm/ns